Test that net can be extended

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>

tests/fixtures/extends/common.yml

@@ -1,6 +1,7 @@
 web:
   image: busybox
   command: /bin/true
+  net: host
   environment:
     - FOO=1
     - BAR=1

tests/fixtures/extends/docker-compose.yml

@@ -11,6 +11,7 @@ myweb:
     BAR: "2"
     # add BAZ
     BAZ: "2"
+  net: bridge
 mydb:
   image: busybox
   command: top